The adsorption separation is very effective for the separation of isomers having an extremely small boiling point difference, or of different components having different structural features, e.g. for the separation of p-xylene from other C8 aromatic isomers, and of n-alkanes from hydrocarbons having other structures.
The separation process by SMB adsorption achieves countercurrent contact of the liquid and solid phases and increases the separation efficiency. U.S. Pat. No. 2,985,589, U.S. Pat. No. 3,201,491, U.S. Pat. No. 3,626,020, U.S. Pat. No. 3,686,342, U.S. Pat. No. 3,997,620 and U.S. Pat. No. 4,326,092 describe the separation device and process by SMB adsorption, and use thereof for separation of p-xylene and m-xylene. Douglas M. Ruthven summarizes in Chemical Engineering Science (1989, v44(5):1011-1038) the principle, development, test and model study and industrial process of the separation process by continuous countercurrent adsorption.
Typical SMB adsorption separation comprises at least two streams of feedstocks, i.e. feedstock (F) and desorbent (D), and at least two streams of the discharged materials, i.e. extract (B) and raffinate (R), wherein the extract is enriched with the target product. The positions at which each stream of the feedstocks is fed into or discharged from the adsorption column are moved periodically, and the feedstocks along with the flow direction of the feedstocks in the adsorption column are in a sequence of the desorbent (D), extract (E), feedstock (F) and raffinate (R). The circulation of feedstocks in the adsorption column makes up an closed-loop. The device for controlling the charging and discharging of the feedstocks to and from the adsorption column may be a rotary valve, or a series of on-off valves.
During the adsorption separation, many streams of feedstocks share the delivery pipelines to be charged into or discharged from the adsorption column. The pipeline discharged into and discharged from a certain bed position of the adsorption column will pass the raffinate (R), feedstock (F), extract (E) and desorbent (D) in turn. The previous residual materials in the pipeline will pollute the materials passing through the pipeline, thereby rendering adverse effect on the separation by SMB adsorption. In particular, when the separation by SMB adsorption is used to product high purity products, the residual feedstocks in the pipelines will pollute the extract.
U.S. Pat. No. 3,201,491 discloses a process for increasing the purity of the product continuously separated by adsorption. As for the circumstance that the residual raw material pollutes the extract, it discloses “charging a flush stream comprising a fluid separable from said feed stock into the fluid inlet next upstream relative to the feed stream inlet in an amount not substantially exceeding the volume of fluid in the line of flow between the feed inlet into the fluid distribution center and the feed inlet to the contact zone receiving said feed stream” The flushing liquid is a desorbent-rich stream removed from a fixed mass of sorbent downstream from the desorbent inlet, or a sorbate-rich stream withdrawn from the farthermost downstream mass of sorbent comprising the desorption zone
U.S. Pat. No. 5,750,820 discloses a multiple grade flush adsorption separation process, which is a process for separating the target product from a multicomponent feedstream, comprising introducing said feedstream through at least one fluid communication conduit into said apparatus; flushing said apparatus having at least one fluid communication conduit with a sufficient quantity of at least one initial flushing medium drawn from a first source and comprising said at least one desired component in an initial concentration, such that feedstream residue is flushed from said apparatus by said at least one initial medium; flushing said at least one fluid communication conduit with a sufficient quantity of a final flushing medium drawn from a second source and comprising said at least one desired component in a final concentration, such that said final concentration is greater than said initial concentration and such that initial medium residue from said conduit is flushed into said conduit into said apparatus by said final medium; and withdrawing said product from said apparatus, wherein said first source is separate from said second source and at least one of said first source and said second source is separated from said apparatus
U.S. Pat. No. 5,972,224 discloses a process and device for improving the purity of a product in a simulated fluid bed, said device comprising a number of beds (A1 to An) of a solid or adsorbent that are contained in at least one adsorption column, a fluid distributor plate (Pi) between each bed, whereby each distributor plate is divided into a number of sectors (P10, P11, P12), whereby each distributor plate sector (Pi) includes at least one distribution chamber that is pierced with openings and a fluid circulation space in the vicinity of said openings of the chamber, and whereby said chamber is connected to a transfer line that extends between the chamber and a point that is located outside of the column; during a period T of the cycle, an injection and a draw-off of each materials into and from a distribution chamber that belongs to different plates are carried out, with the process being characterized in that, at an appropriate flow rate, a fluid volume is permanently circulated that circulates in the column in a bypass line that connects different chambers of the distributor plates; the flushing liquid has a composition close to the circulating fluid. The object thereof lies in avoiding greater composition difference between the flushing materials introduced from outside and the materials in the adsorption column to cause disturbance to the separation process. However, such solution will also cause a problem, i.e. a stream of the feedstocks not passing through the adsorption chamber, which is equivalent to a stream of channeling in the adsorption bed, and is adverse to the adsorption separation.
U.S. Pat. No. 6,004,518 discloses a high-purity simulated moving bed adsorptive separation apparatus, comprising a series of individual adsorbent chambers adapted for retaining a bed of adsorbent, a series of fluid transfer lines interconnecting the adsorbent chambers and also allowing passage of feedstock and desorbent streams into the apparatus and removal of extract and raffinate streams from the apparatus, a series of valves for controlling the flow of fluids through the fluid transfer lines and between adsorbent chambers, with a separate set of valves being associated with each adsorbent chamber and with said valves being characterized in that two ports of each valve which controls the flow of the feed stream into a specific adsorbent chamber are connected to the fluid transfer line which connects this specific adsorbent chamber with a next upstream adsorbent chamber; whereby fluid discharged from the next upstream adsorbent chamber may flow through the valve and flush the feed stream from the valve and from a conduit leading from this valve to the specific adsorbent chamber. However, each bed of adsorbent is required to be separate, and the circulating fluid entering the next bed from the previous bed needs to pass through the lines and valve.